Class 8 Science Force and Pressure Notes and Solutions

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Force and Pressure

 

Force

A pull or push over an object is called force.
Example – to open a drawer, one has to pull it; and to close the drawer one has to push it. Thus, in both the conditions a person applies force by pull or push.
To kick a ball, one has to push it, i.e. a force is applied.
To ride a bicycle one has to push the pedal. In this condition, force is applied while pushing the pedal.
Forces are due an interaction:
To apply a force over an object interaction between object and source of force is necessary.
Example –
To open a drawer one has to pull by holding it, drawer will not open otherwise .
To kick a ball one has to touch it, otherwise force will not be applied over the ball.

Direction and Magnitude of Force:

The measure of amount or strength of force is called the magnitude of force. Thus, strength or amount of force is expressed in terms of magnitude.

Condition when more than two forces are applied over an object:

Forces applied in same direction:

When more than one force is applied in the same direction, the total force is the magnitude of both the forces.
Example – It is difficult to push a car by single person, but two or more persons can collectively push the same car easily. This is because when more than one person tries to push the car, magnitudes of forces applied by each of the person are summed up and applied over than car, and car is pushed easily because of greater magnitude of force.
That means if three units of force are applied over an object by each of the two persons,
The total unit of force = sum of both forces = 3 unit + 3 unit = 6 unit

Forces applied in opposite direction:

When two forces are applied over an object from opposite directions, the total effective force is the difference of two forces.

Example –

(a) When two persons are pushing a box in opposite directions and the magnitude of both the forces is equal, then box will not move, because the difference of two forces will be equal to zero.
Let a person applying 4 unit of force in one direction and other person is also applying 4 unit of force in opposite direction on the same wooden box.
Therefore, the magnitude of total force = 4 unit – 4 unit = 0
(b) If one person is applying a force of 4 unit in one direction and another person is applying a force of 6 unit in opposite direction,
Then total magnitude of force = 6 unit – 4 unit = 2 unit
Thus, force will act in the direction of larger magnitude of force.

Force can change the state of motion:

State of rest:

An object is called in the state of rest when it is not moving. This means a stationary object is called in the state of rest. For example – a building, an electric pole, a ball kept over the ground and not moving, etc.

State of motion:

A moving object is called in the state of motion. For example – a moving car, a moving ball, etc.
A force can speed up a moving object. Force can decrease the speed of a moving object. Force can stop a moving object. Force can change the direction of a moving object. Force can move a stationary object.

Example –

In the game of cricket, when a batsman pushes a fast moving ball in the same direction of the movement of ball, the speed of the ball is increased.
When a batsman pushes a fast moving ball in the opposite direction the speed of ball decreases; because of force being applied from his bat.
When a player in the game of football kicks the ball to his mate, he changes the direction of the moving football by applying force through his kick.
A goalkeeper stops the football going towards the goal post by applying a force by his hand.

Force can change the shape of an object:

When you apply force on an inflated balloon; using your hand from both sides, the force of pressure changes the shape of balloon.
You mom change the shape of dough into a bread by applying force with a rolling pin.
A blacksmith changes the shape of an iron rod by applying force using a hammer.
When a rubber band is stretched in opposite directions, its shape is changed.

Force and Pressure

Kinds of Force

Contact Force

Force that comes into action after the interaction between objects is called contact force. Contact force acts on the point of contact. For contact force; interaction between objects is necessary. Example; pushing a car, opening a drawer, kicking a ball, etc. In these examples, interaction between objects is necessary.
Muscular force, force of friction, etc. are types of contact force.

Muscular Force: -

Force caused by the action of muscles is called muscular force. In other words, force resulting because of action of muscle is called muscular force. Muscular force is applied only after interaction with the object. Hence it is a type of contact force.

Example : –

While kicking a ball, the player applies force over the ball using his leg muscles.
While opening a drawer, you pull the drawer holding it by hand. In this action, muscles of your hand apply the force.
While pulling a cart, the horse or the ox applies the force of its muscles.

Friction:

Force acting between the surfaces of two objects is called the force of friction. Force of friction always acts in the opposite direction of the movement of object. Force of friction is acting over all the moving objects.

Example -

A moving football stops after going to a certain distance. This happens because of force of friction between the surface of ground and the surface of football.
A moving boat stops after some distance because of friction between the surface of water and the surface of boat.
Since force of friction comes into action only after interaction between two objects, thus, it is a type of contact force.

Non – contact Force

Force that comes into action without interaction between two objects is called non-contact force. For example; a magnet can pull an iron nail from a distance.
Magnetic force, Electrostatic force and Gravitational force are examples of non-contact force.

Magnetic Force:

Force exerted by a magnet on another magnet or on magnetic substances is called magnetic force. A magnet can exert force even without coming in contact, thus it is a non-contact force.
Example – A magnet attracts the opposite pole of another magnet and repels the similar pole of another magnet.

Electrostatic Force:

Force exerted by a charged body is called electrostatic force. A charged body attracts an uncharged body. A positively charged body attracts a negatively charged body and repels a positively charged body without coming in contact, thus it is a non-contact force.

Gravitational Force:

Force exerted by earth, moon, sun and other planets is called gravitational force. Earth attracts all objects towards it. Similarly, all other planets along with moon attract all objects towards them. Since, earth attracts all objects even without coming in contact, thus gravitational force is a non-contact force.

Example –

When anything is released from a height, it falls over the ground because of gravitational pull of earth.

Pressure:

A force applied over the unit area of a surface is called pressure. In other words, force per unit area is called pressure.
Pressure = Force / Area
In this case, area is indirectly proportional to the pressure, thus pressure decreases with increase in area and increases with decrease in area.

Example:

While cutting an apple, we need to use the sharp edge of the knife. Using the blunt edge of knife shall not serve the purpose. The blunt edge of knife has larger surface area than the sharp edge. Because of smaller surface area; more pressure can be applied through the sharp edge of the knife and something can be easily cut.
While putting a nail into a wooden board, the pointed end of the nail is kept at the front. The pointed end of the nail has very small surface area and this enables us to apply a greater pressure with the applied force.
School bags have broad shoulder straps. Because of broad shoulder straps, the pressure because of the weight of the school bag is distributed over a larger area and it becomes easier to carry the bag.

Pressure exerted by Liquid and Gas

Similar to solid, liquid and gas also exert pressure. Liquid and gas exert pressure on inner walls of the container in which they are kept.

Example –

Water starts leaking if there is a pore in the bottle. This happens because water exerts pressure over the walls of the bottle.
Water flows from higher level to lower level of the ground. This happens because of pressure exerted by water.
When air is filled, a rubber balloon gets inflated from all sides. This happens because air, which is a gas, exerts pressure over inner walls of the balloon.

Atmospheric Pressure:

Our atmosphere is made of air. Since air is mixture of gases and gas exerts pressure, thus air exerts pressure. Pressure exerted by air (present in atmosphere) is called atmospheric pressure.
Atmosphere of 10cm X 10cm X 10cm exerts about 1000 kg of pressure.
We do not feel the atmospheric pressure over us because the pressure inside our body nullifies the atmospheric pressure.

Example –

A pressed rubber sucker on a plane surface does not come off because of atmospheric pressure.
When you go to hills and mountains, it is difficult to breather. This happens because of low atmospheric pressure.
The ink of a fountain pen spills out in aero plane, because of low pressure at higher altitude.
At higher altitude, lentils (pulses) take longer to get cooked. This happens because of low atmospheric pressure at higher altitude. Because of low atmospheric pressure, water boils at a lower temperature and that temperature is not enough for cooking the pulses.

Important terms

Force - A pull or push over an object is called force.
Magnitude of Force: Strength or amount of force is expressed in terms of magnitude.
Contact Force: Force that comes into action after the interaction between objects is called contact force.
Muscular Force: Force caused by the action of muscles is called muscular force.
Friction: Force acting between the surfaces of two objects is called force of friction.
Non – contact Force: Force that comes into action without interaction between two objects is called non-contact force.
Magnetic Force: Force exerted by a magnet on other magnet or on magnetic substances is called magnetic force.
Electrostatic Force: Force exerted by a charged body is called electrostatic force.
Gravitational Force: Force exerted by earth, moon, sun and other planets to pull an object towards them, is called gravitational force.
Pressure: A force applied over the unit area of a surface is called pressure.
Atmospheric Pressure: Pressure exerted by air present in atmosphere is called atmospheric pressure.

Force and Pressure

Kinds of Force

Contact Force

Force that comes into action after the interaction between objects is called contact force. Contact force acts on the point of contact. For contact force; interaction between objects is necessary. Example; pushing a car, opening a drawer, kicking a ball, etc. In these examples, interaction between objects is necessary.
Muscular force, force of friction, etc. are types of contact force.

Muscular Force: -

Force caused by the action of muscles is called muscular force. In other words, force resulting because of action of muscle is called muscular force. Muscular force is applied only after interaction with the object. Hence it is a type of contact force.

Example : –

While kicking a ball, the player applies force over the ball using his leg muscles.
While opening a drawer, you pull the drawer holding it by hand. In this action, muscles of your hand apply the force.
While pulling a cart, the horse or the ox applies the force of its muscles.

Friction:

Force acting between the surfaces of two objects is called the force of friction. Force of friction always acts in the opposite direction of the movement of object. Force of friction is acting over all the moving objects.

Example -

A moving football stops after going to a certain distance. This happens because of force of friction between the surface of ground and the surface of football.
A moving boat stops after some distance because of friction between the surface of water and the surface of boat.
Since force of friction comes into action only after interaction between two objects, thus, it is a type of contact force.

Non – contact Force

Force that comes into action without interaction between two objects is called non-contact force. For example; a magnet can pull an iron nail from a distance.
Magnetic force, Electrostatic force and Gravitational force are examples of non-contact force.

Magnetic Force:

Force exerted by a magnet on another magnet or on magnetic substances is called magnetic force. A magnet can exert force even without coming in contact, thus it is a non-contact force.
Example – A magnet attracts the opposite pole of another magnet and repels the similar pole of another magnet.

Electrostatic Force:

Force exerted by a charged body is called electrostatic force. A charged body attracts an uncharged body. A positively charged body attracts a negatively charged body and repels a positively charged body without coming in contact, thus it is a non-contact force.

Gravitational Force:

Force exerted by earth, moon, sun and other planets is called gravitational force. Earth attracts all objects towards it. Similarly, all other planets along with moon attract all objects towards them. Since, earth attracts all objects even without coming in contact, thus gravitational force is a non-contact force.

Example –

When anything is released from a height, it falls over the ground because of gravitational pull of earth.

Pressure:

A force applied over the unit area of a surface is called pressure. In other words, force per unit area is called pressure.
Pressure = Force / Area
In this case, area is indirectly proportional to the pressure, thus pressure decreases with increase in area and increases with decrease in area.

Example:

While cutting an apple, we need to use the sharp edge of the knife. Using the blunt edge of knife shall not serve the purpose. The blunt edge of knife has larger surface area than the sharp edge. Because of smaller surface area; more pressure can be applied through the sharp edge of the knife and something can be easily cut.
While putting a nail into a wooden board, the pointed end of the nail is kept at the front. The pointed end of the nail has very small surface area and this enables us to apply a greater pressure with the applied force.
School bags have broad shoulder straps. Because of broad shoulder straps, the pressure because of the weight of the school bag is distributed over a larger area and it becomes easier to carry the bag.

Pressure exerted by Liquid and Gas

Similar to solid, liquid and gas also exert pressure. Liquid and gas exert pressure on inner walls of the container in which they are kept.

Example –

Water starts leaking if there is a pore in the bottle. This happens because water exerts pressure over the walls of the bottle.
Water flows from higher level to lower level of the ground. This happens because of pressure exerted by water.
When air is filled, a rubber balloon gets inflated from all sides. This happens because air, which is a gas, exerts pressure over inner walls of the balloon.

Atmospheric Pressure:

Our atmosphere is made of air. Since air is mixture of gases and gas exerts pressure, thus air exerts pressure. Pressure exerted by air (present in atmosphere) is called atmospheric pressure.
Atmosphere of 10cm X 10cm X 10cm exerts about 1000 kg of pressure.
We do not feel the atmospheric pressure over us because the pressure inside our body nullifies the atmospheric pressure.

Example –

A pressed rubber sucker on a plane surface does not come off because of atmospheric pressure.
When you go to hills and mountains, it is difficult to breather. This happens because of low atmospheric pressure.
The ink of a fountain pen spills out in aero plane, because of low pressure at higher altitude.
At higher altitude, lentils (pulses) take longer to get cooked. This happens because of low atmospheric pressure at higher altitude. Because of low atmospheric pressure, water boils at a lower temperature and that temperature is not enough for cooking the pulses.

Important terms

Force - A pull or push over an object is called force.
Magnitude of Force: Strength or amount of force is expressed in terms of magnitude.
Contact Force: Force that comes into action after the interaction between objects is called contact force.
Muscular Force: Force caused by the action of muscles is called muscular force.
Friction: Force acting between the surfaces of two objects is called force of friction.
Non – contact Force: Force that comes into action without interaction between two objects is called non-contact force.
Magnetic Force: Force exerted by a magnet on other magnet or on magnetic substances is called magnetic force.
Electrostatic Force: Force exerted by a charged body is called electrostatic force.
Gravitational Force: Force exerted by earth, moon, sun and other planets to pull an object towards them, is called gravitational force.
Pressure: A force applied over the unit area of a surface is called pressure.
Atmospheric Pressure: Pressure exerted by air present in atmosphere is called atmospheric pressure.

Force and Pressure

Solution of NCERT Exercise

Question: 1. Give two examples each of situations in which you push or pull to change the state of motion of objects.

Answer: -

Push: -

To close a drawer.

To move a wooden box on floor.

Pull:

To open a drawer

A horse pulling a cart.

Question: 2. Give two examples of situations in which applied force causes a change in the shape of an object.

Answer:

Making of bread from dough

Stretching of a rubber band.

Question: 3. Fill in the blanks in the following statements:
(a) To draw water from a well we have to ——— at the rope.

Answer: Pull

(b) A charged body ———— an uncharged body towards it.

Answer: Attracts

(c) To move a loaded trolley we have to ———— it.

Answer: Push or Pull

(d) The north pole of a magnet ———— the north pole of another magnet.

Answer: repels

Question: 4. An archer stretches her bow while taking aim at the target. She then releases the arrow, which begins to move towards the target. Based on this information fill up the gaps in the following statements using the following terms:
muscular, contact, non-contact, gravity, friction, shape, attraction
(a) To stretch the bow, the archer applies a force that causes a change in its ________.

Answer: Shape

(b) The force applied by the archer to stretch the bow is an example of — —— force.

Answer: Muscular

(c) The type of force responsible for a change in the state of motion of the arrow is an example of a ———— force.

Answer: Contact

(d) While the arrow moves towards its target, the forces acting on it are due to ———— and that due to ———— of air.

Answer: gravity, friction

Question: 5. In the following situations identify the agent exerting the force and the object on which it acts. State the effect of the force in each case.
(a) Squeezing a piece of lemon between the fingers to extract its juice.

Answer:

Agent exerting the force – Fingers

Object on which force acts – Lemon

Effect of force – Change in shape of lemon

(b) Taking out paste from a toothpaste tube.

Answer:

Agent exerting the force – Fingers

Object on which force acts – toothpaste tube

Effect of force – Change in shape of toothpaste tube.

(c) A load suspended from a spring while its other end is on a hook fixed to a wall.

Answer:

Agent exerting the force – Load

Object on which force acts – Spring

Effect of force – Change in shape of spring.

(d) An athlete making a high jump to clear the bar at a certain height.

Answer:

Agent exerting the force – Muscles of Athlete

Object on which force acts – Athlete

Effect of force – Change of state of motion of athlete

Question: 6. A blacksmith hammers a hot piece of iron while making a tool. How does the force due to hammering affect the piece of iron?

Answer:

Force due to hammering changes the shape of the piece of iron.

Question: 7. An inflated balloon was pressed against a wall after it has been rubbed with a piece of synthetic cloth. It was found that the balloon sticks to the wall. What force might be responsible for the attraction between the balloon and the wall?

Answer: Electrostatic force

Question: 8. Name the forces acting on a plastic bucket containing water held above ground level in your hand. Discuss why the forces acting on the bucket do not bring a change in its state of motion.

Answer: In this case, muscular force is acting in upward direction and the force of gravity is acting in downward direction. Both forces are acting in opposite directions to each other and hence nullify the effect of each other. Due to this, there is no change in the state of motion of the bucket.

Question: 9. A rocket has been fired upwards to launch a satellite in its orbit. Name the two forces acting on the rocket immediately after leaving the launching pad.

Answer: Gravitational force and atmospheric pressure.

Question: 10. When we press the bulb of a dropper with its nozzle kept in water, air in the dropper is seen to escape in the form of bubbles. Once we release the pressure on the bulb, water gets filled in the dropper. The rise of water in the dropper is due to
(a) pressure of water
(b) gravity of the earth
(c) shape of rubber bulb
(d) atmospheric pressure

Answer: (d) Atmospheric pressure